1. Field of Invention
The present invention is generally directed at engine control strategies for internal combustion engines, and in particular to engines having direct fuel injection systems. Furthermore, the present invention is equally applicable for both single and dual fluid direct fuel injection systems.
2. Description of the Related Art
A majority of fuel injected engines/vehicles currently produced by the major engine and vehicle manufacturers of the world are of the conventional manifold or port fuel injection (MPI/PFI) type. However, with the ongoing efforts to extract better performance from internal combustion engines and the emerging requirements to meet stringent emission laws, more and more engine manufacturers are investing and developing direct fuel injection engine technologies. Such direct injection (DI) engine technologies are considered by some to be the next evolutionary step for internal combustion engines and examples of automotive vehicles incorporating DI engines are in fact already available to the consumer in certain automotive markets.
Although the progression to DI engine technology may, depending upon the specific engine application, result in some short-term increase in the cost of automotive engines, it is believed that the numerous advantages that DI engines have over MPI/PFI engines will outweigh any such increase in cost. In particular, there are certain emissions and performance benefits which can be realised by adopting a DI fuel system in place of an MPI fuel system.
In regard to emissions benefits, one significant advantage of DI engines over MPI/PFI engines is that, under certain operating conditions, relatively lower exhaust emissions are typically produced by DI engines. This arises in part from the reduced level of wall wetting which generally occurs in a DI engine. As this leads to more of the metered fuel quantity being burned in the combustion chamber of the engine, a noticeable decrease in the production of hydrocarbon (HC) emissions typically results during certain portions of the engine operating load range.
In particular, reduced wall wetting, together with the generally lower crank fuelling required in a DI engine contributes to the reduction of cold start HC emissions. Reduced wall wetting also leads to decreased HC emissions during engine transients and also enables better reinstatement of combustion following cylinder deactivation events, this invariably leading to lower HC emissions.
Almost all current design MPI and DI engines include a catalytic converter or exhaust gas after treatment system of some nature located in the exhaust system of the vehicle. The catalytic converter typically acts to convert undesirable exhaust emissions such as hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) into non-harmful substances such as carbon dioxide, nitrogen, oxygen and water. As alluded to hereinbefore, the recent and future introduction of increasingly stringent internal combustion engine emissions legislation around the world, such as the proposed US ULEV II & SULEV emissions regulations, has resulted in increasing pressure on engine and vehicle manufacturers to reduce engine emissions. The proposed SULEV emissions are particularly stringent with respect to HC and NOx emissions from vehicles.
In meeting these stringent emissions standards most MPI and PFI vehicles suffer a fuel consumption penalty even though overall tail pipe emission levels decrease. This increase in fuel consumption arises for various reasons including increased engine hardware requirements that serve to increase the level of parasitic loading on the engine, increased fuel consumption due to the catalyst light-off strategies used and an increase in fuel consumption that arises when the engine is calibrated to produce reduced levels of hydrocarbon emissions and NOx emissions.
Insofar as engine performance is concerned, DI engines offer a number of advantages over MPI/PFI engines. For example, better wide-open throttle (WOT) performance can be achieved in DI engines because of the charge cooling effect which is experienced. That is, as there is less wall wetting in a DI engine, a greater quantity of fuel effectively mixes with and vaporizes the air charge in the combustion chamber of the engine. This has the effect of cooling the overall fuel-air charge and hence increasing the volumetric efficiency of the engine which in turn leads to improved full load performance.
This charge cooling effect further serves to reduce the peak temperatures and pressures attained in the combustion chamber and thus leads to a reduction in the tendency for knock to occur. This knock suppression effect in turn reduces the octane sensitivity of the engine such that a wider range of fuels can be utilised with the engine. Alternatively, such characteristics can enable DI engines to operate at higher compression ratios resulting in improved fuel economy.
Still further, the cam timing of a DI engine can be configured to run with a greater degree of valve overlap because of the increased combustion stability provided by a DI engine, in particular when operating with a stratified charge. Such valve overlap is typically used to permit a certain degree of exhaust residuals to be present in the fresh fuel-air charge which, for certain points of the engine operating load range, may provide for reduced pumping work by the engine. Hence, greater overlap may enable the degree of residuals to be increased resulting in better fuel consumption, particularly through the low to mid load ranges.
A further advantage of DI engines over MPI/PFI engines is the increased level of responsiveness of the engine due to the in-cylinder fuel delivery. This effectively results in a reduced degree of lag between the operator requesting a load/speed charge and this change being effected. This leads to improved performance and combustion efficiencies.
There are therefore many advantages to using a DI engine over more conventional MPI/PFI engines in vehicle applications.
In this regard, the Applicant has developed and applied such DI fuel system technology to numerous different engine applications. More specifically, the Applicant has developed and commercialised various air-assist or dual fluid fuel injection systems to inject fuel directly into the combustion chambers of an engine. Such systems typically utilise compressed gas during each injection event to entrain and atomise a metered quantity of fuel for delivery into the combustion chambers of the engine. One such dual fluid injection system is described in the Applicant's U.S. Pat. No. RE36768, the details of which are incorporated herein by reference. Generally, a source of compressed gas, for example an air compressor, is required for these fuel injection systems to operate satisfactorily. The term “air” is used to refer not only to atmospheric air, but may also refer to other gases including air and exhaust gas or fuel vapour mixtures. In operation, such dual fluid fuel injection systems typically rely on the existence of a differential pressure between the fuel which is metered for subsequent delivery and the compressed gas, typically air, which is used to deliver the fuel to the engine. In this regard, it is normal that the fuel pressure is slightly higher than the air pressure such that the fuel may be metered into a volume of compressed gas in a manner akin to that described in U.S. Pat. No. RE 36768.
A significant portion of the current activity taking place in respect of DI engine technology development is in relation to lean burn engines. That is, a number of DI engines currently being developed have the capability to run lean over a significant portion of the engine operating load range, such lean operation allowing significant fuel consumption reductions to be realised. Such DI engines typically operate with a stoichiometric air/fuel ratio throughout a majority of the remaining portion of the engine operating load range. Lean operation is typically associated with the provision of a stratified fuel-air charge in the combustion chamber whilst stoichiometric operation is typically associated with a homogenous fuel-air charge. Both single fluid and dual fluid DI engines can provide a stratified charge, particularly at lower engine loads, which can lead to improved fuel economy and reduced exhaust emissions.
There are however certain challenges to be faced when effecting lean burn operation. In particular, lean operation typically results in the formation of NOx emissions which are more difficult to be reduced by after treatment systems. In this regard, conventional three-way catalytic converters (TWC's) have been found to be unsatisfactory for efficiently treating such NOx emissions produced during such lean burn operation. One present way of addressing this is by incorporating a further Lean NOx Trap (LNT) catalyst which acts to adsorb NOx gases emitted from the engine until the engine conditions are more favourable for the trapped NOx gases to be converted by the exhaust gas treatment system. The favourable engine condition is typically when the engine is operating with a rich or stoichiometric air/fuel ratio. Accordingly, in current systems incorporating an LNT, it has been found necessary for the engine to temporarily run with a rich air/fuel ratio to promote desorption of NOx stored/trapped on the LNT.
There have however been found to exist certain issues with the use of LNTs, not the least of which is added cost. Due to the precious metal loadings necessary on such LNTs, significant cost increases are likely to be incurred for any DI engine incorporating such an LNT. Furthermore and equally prohibitive is the sensitivity of an LNT to sulphur. Currently available fuels typically contain a significant proportion of sulphur which has been found to effectively “poison” an LNT such that after a certain period of time it is no longer effective in trapping and treating NOx. Hence, unless low sulphur fuels can be introduced in certain markets, the use of an engine incorporating an LNT may be seen as being problematic in these markets. In many countries including the United States of America, low sulphur content fuel is still not readily available. It is therefore not possible to effectively use an LNT on vehicles to be driven in the U.S. and other such countries where the emission regulations require strict control of NOx emissions
Under these circumstances it would be preferable to be able to operate the DI engine such that it is only necessary to use a conventional catalytic converter such as a TWC without the need for any LNT converter. In this way, the advantages of adopting DI could be immediately realized without the need to address the cost and durability problems associated with an LNT.